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Small-molecule dyes are generally designed based on well-understood electronic effects. However, steric hindrance can promote excited-state geometric relaxation, increasing the difference between the positions of absorption and emission bands (the Stokes shift). Accordingly, we hypothesized that sterically induced central ring puckering in xanthene dyes could be used to systematically increase their Stokes shift. Through a combined experimental/quantum-chemical approach, we screened a group of (9-acylimino)-pyronin dyes with a perturbed central ring geometry. Our results showed that an atom with sp 3 hybridization in position 10 of (9-acylimino)-pyronins induces central ring puckering and facilitates excited-state geometric relaxation, thereby markedly enhancing their Stokes shifts (by up to ~2000 cm -1 ). Thus, we prepared fluorescent (9-acylimino)-pyronin pH sensors, which showed a Stokes shift disparity between acid and base forms of up to ~8700 cm -1 . Moreover, the concept of ring puckering-enhanced Stokes shift can be applied to a wide range of xanthene analogues found in the literature. Therefore, central ring puckering may be reliably used as a strategy for enhancing Stokes shifts in the rational design of dyes., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) dyes are extensively used in various applications of their triplet states, ranging from photoredox catalysis, through triplet sensitization to photodynamic therapy. However, the rational design of BODIPY triplet chromophores by ab initio modelling is limited by their strong interactions of spin, electronic and vibrational dynamics. In particular, spin-vibronic coupling is often overlooked when estimating intersystem crossing (ISC) rates. In this study, a combined experimental and theoretical approach using spin-vibronic coupling to correctly describe ISC in BODIPY dyes was developed. For this purpose, seven π-extended BODIPY derivatives with iodine atoms in different positions were examined. It was found that the heavy-atom effect of iodine atoms is site specific, causing high triplet yields in only some positions. This site-specific ISC was explained by El-Sayed rules, so both the contribution and character of the molecular orbitals involved in the excitation must be considered when predicting the ISC rates. Overall, the rational design of BODIPY triplet chromophores requires using (i) the high-quality electronic structure theory, including both static and dynamical correlations; and (ii) the two-component wave function Hamiltonian, and rationalizing; and (iii) ISC based on the character of the molecular orbitals of heavy atoms involved in the excitation, expanding El-Sayed rules beyond their traditional applications., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

N-Trifluoromethylated organics may be applied in drug design, agrochemical synthesis, and materials science, among other areas. Yet, despite recent advances in the synthesis of aliphatic, cyclic and heterocyclic N-trifluoromethyl compounds, no strategy based on trifluoromethyl nitrene has hitherto been explored. Here we describe the formation of triplet trifluoromethyl nitrene from azidotrifluoromethane, a stable and safe-to-use precursor, by visible light photocatalysis. The addition of CF 3 N to alkenes via biradical intermediates afforded previously unknown aziridines substituted with trifluoromethyl group on the nitrogen atom. The obtained aziridines were converted into either N-trifluoromethylimidazolines, via formal [3+2] cycloaddition with nitriles, mediated by a Lewis acid, or into N-trifluoromethylaldimines, via ring opening and aryl group migration mediated by a strong Brønsted acid. Our findings open new opportunities for the development of novel classes of N-CF 3 compounds with possible applications in the life sciences., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Organic solar cells have been continuously studied and developed through the last decades. A major step in their development was the introduction of fused-ring non-fullerene electron acceptors. Yet, beside their high efficiency, they suffer from complex synthesis and stability issues. Perylene-based non-fullerene acceptors, in contrast, can be prepared in only a few steps and display good photochemical and thermal stability. Herein, we introduce four monomeric perylene diimide acceptors obtained in a three-step synthesis. In these molecules, the semimetals silicon and germanium were added in the bay position, on one or both sides of the molecules, resulting in asymmetric and symmetric compounds with a red-shifted absorption compared to unsubstituted perylene diimide. Introducing two germanium atoms improved the crystallinity and charge carrier mobility in the blend with the conjugated polymer PM6. In addition, charge carrier separation is significantly influenced by the high crystallinity of this blend, as shown by transient absorption spectroscopy. As a result, the solar cells reached a power conversion efficiency of 5.38 %, which is one of the highest efficiencies of monomeric perylene diimide-based solar cells recorded to date., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Gain of aromaticity or relief of antiaromaticity along a reaction path are important factors to consider in mechanism studies. Analysis of such changes along potential energy surfaces has historically focused on reactions in the electronic ground state (S 0 ), but can also be used for excited states. In the lowest ππ* states, the electron counts for aromaticity and antiaromaticity follow Baird's rule where 4n π-electrons indicate aromaticity and 4n+2 π-electrons antiaromaticity. Yet, there are also cases where Hückel's rule plays a role in the excited state. The electron count reversals of Baird's rule compared to Hückel's rule explain many altered physicochemical properties upon excitation of (hetero)annulene derivatives. Here we illustrate how the gain of excited-state aromaticity (ESA) and relief of excited-state antiaromaticity (ESAA) have an impact on photoreactivity and photostability. Emphasis is placed on recent findings supported by the results of quantum chemical calculations, and photoreactions in a wide variety of areas are covered., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Catalyst recovery is an integral part of photoredox catalysis. It is often solved by adding another component-a sacrificial agent-whose role is to convert the catalyst back into its original oxidation state. However, an additive may cause a side reaction thus decreasing the selectivity and overall efficiency. Herein, we present a novel approach towards chemoselective photooxidation reactions based on suitable solvent-acetonitrile acting simultaneously as an electron acceptor for catalyst recovery, and on anaerobic conditions. This is allowed by the unique properties of the catalyst, 7,8-dimethoxy-3-methyl-5-phenyl-5-deazaflavinium chloride existing in both strongly oxidizing and reducing forms, whose strength is increased by excitation with visible light. Usefulness of this system is demonstrated in chemoselective dehydrogenations of 4-methoxy- and 4-chlorobenzyl alcohols to aldehydes without over-oxidation to benzoic acids achieving yields up to 70 %. 4-Substituted 1-phenylethanols were oxidized to ketones with yields 80-100 % and, moreover, with yields 31-98 % in the presence of benzylic methyl group, diphenylmethane or thioanisole which are readily oxidized in the presence of oxygen but these were untouched with our system. Mechanistic studies based on UV-Vis spectro-electrochemistry, EPR and time-resolved spectroscopy measurements showed that the process involving an electron release from an excited deazaflavin radical to acetonitrile under formation of solvated electron is crucial for the catalyst recovery., (© 2022 Wiley-VCH GmbH.)

Metal ions can have beneficial effects on photoinduced electron transfer. Merging such metal-ion-coupled electron transfer (MCET) with consecutive photoinduced electron transfer (conPET) enables the one-electron reduction of chlorobenzene with blue light in the presence of diisopropylethylamine as an electron donor. The presence of the metal ions extends the substrate scope of the photoredox catalysis to extreme reduction potentials (beyond -3 V vs. SCE)., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Alkyl- and aryl vinyl sulfones were obtained by eosin Y (EY)-mediated visible-light photooxidation of sulfinate salts and the reaction of the resulting S-centered radicals with alkenes. Optimized reaction conditions, the sulfinate and alkene scope, and X-ray structural analyses of several reaction products are provided. A detailed spectroscopic study explains the reaction mechanism, which proceeds through the EY radical cation as key intermediate oxidizing the sulfinate salts., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)